AB-INITIO CALCULATIONS ON THE INTERCONVERSION OF HOMOCUB-1(9)-ENE ANDHOMOCUB-9-YLIDENE

Calculations which include dynamic electron correlation beyond the CAS SCF level have been carried out for the interconversion of homocub-1(9 )-ene (1) and homocub-9-ylidene (2). The geometry of the transition st ate was also located at the (4/4)CASSCF/6-31G level and was found to differ significantly from the previously published (2/2)CASSCF/3-21G g eometry. In agreement with experiment, calculations at the (2/2)CASSDC I + Q/6-31G level find 1 and 2 to have free energies at 298 K that di ffer by only similar to 0.1 kcal mol(-1). However, the activation ener gy for the rearrangement of 1 to 2 of E-a = 12.9 kcal mol(-1), compute d at this level of theory, is considerably higher than the experimenta l estimate of E-a less than or equal to 5 kcal mol(-1). Additional cal culations in which the size of the basis set was increased or in which the active space was expanded to 4 electrons in 4 orbitals suggest th at the calculated E-a may be reduced to as low as 8 kcal mol(-1). Howe ver, there is no indication that higher level calculations would give a Value of E-a as low as 5 kcal mol(-1). Possible reasons for this app arent disagreement between the calculations and the experiments on the rate of equilibration of 1 and 2 are discussed.